Leptestiina veturna, Zhan & Jin, 2005

Zhan, Renbin & Jin, Jisuo, 2005, Brachiopods from the Middle Ordovician Shihtzupu Formation of Yunnan Province, China, Acta Palaeontologica Polonica 50 (2), pp. 365-393 : 385

Leptestiina veturna sp. nov.

Figs. 12N, 13A–I ; Table 13.

Derivation of name: From the Latin veturnus, old, referring to the lowest known range of the genus represented by the new species.

Holotype: NIGP 139147 ( Fig. 13C ), ventral internal mould.

Type locality: Shizigou Valley near the Weixin County town , Yunnan Province .

Type horizon: Upper part of Shihtzupu Formation.

Material.—Thirteen ventral internal and four external, two dorsal internal and nine external moulds. Collection AFI1680.

Diagnosis.—Small, weakly transverse, strongly concavoconvex shells of Leptestiina , with relatively sparse accentuated costae. Description.—Shell small, concavoconvex, subsemicircular to subcircular; maximum width along hinge line. Cardinal extremities acute to rounded. Anterior commissure rectimarginate. Ventral valve about three−quarters as long as wide, strongly convex, deepest in central part; visceral area more convex than marginal area of shell; interarea high, about one−eighth length of shell, apsacline with planar or slightly curved surface; delthyrium relatively wide, covered by weakly arched pseudodeltidium in its posterior two−thirds ( Fig. 13C 2 ). Dorsal valve about three−fifths as long as wide, deeply concave, with greatest concavity at junction between visceral area and geniculation; interarea low, shorter than one−tenth length of shell, anacline; notothyrium narrow, short, covered by arched chilidium ( Fig. 13H 2 ). Unequal parvicostellae; 7–9 accentuated costae, with finer costellae inserting three times; about 9–11 fine costellae between two adjacent costae at shell anterior margin. Concentric fila evenly spaced, about 15–16 per mm.

Teeth small, wedge−shaped; dental plates short, high, subparallel, continuous with anterior bounding ridge of muscle field. Muscle field subtriangular to subpentagonal, about one−fifth length and width of shell, elevated anteriorly; adductor scars small, narrow, mainly in posteromedial part of muscle field, separated from diductor scars by pair of thin plates in some shells ( Fig. 13D ); diductor scars much larger, subcircular, surrounding adductor scars laterally and anteriorly. Mantle canal system saccate; vascula media originating from anterior ends of diductor scars, with long, straight, weakly divergent main trunks. Bema relatively large, well−developed; dorsal platform marked by weak, discrete septules. Discussion.—The convexity of ventral valves has a certain degree of variation: the visceral area is usually much more convex than the shell marginal area, giving the valve a galeate shape (e.g., Fig. 13E 1 ), but some shells are almost evenly convex ( Figs. 12N, 13C 1 ).

There are seven species assigned to Leptestiina , ranging from early Caradocian to mid−Ashgillian ( Cocks and Rong 1989, 2000). In China, previously known forms of the genus are associated with the Foliomena fauna in the upper Pingliang (upper Caradocian) and the Linhsiang (lower Ashgillian) formations ( Fu 1982; Rong et al. 1994). The new species of late Darriwilian age is probably the oldest known of the genus. In comparison, the type species, L. prantli Havlíček, 1952 from the Králův Dvůr Formation (middle Ashgillian) of Bohemia, can be distinguished by its more transverse shell, more numerous costae, and lower shell convexity. Leptestiina cf. prantli from the Linhsiang Formation of western Guizhou Province (Rong et al. 1994) has a strongly transverse shell.

Figures

Fig. 13. A–I. Leptestiina veturna sp. nov. A. NIGP139145, paratype, ventral internal mould. B. NIGP139146, paratype, ventral internal mould. C. NIGP139147, holotype, ventral internal mould (C1) and latex cast (C2). D. NIGP139148, paratype, ventral internal mould. E. NIGP139149, paratype, ventral internal mould (E1) and latex cast (E2). F. NIGP139150, paratype, dorsal internal mould. G. NIGP139151, paratype, three dorsal external moulds, with dorsal external mould of Leptellinidae gen. et sp. nov. (to be erected by Zhan and Jin in press) (second from right). H. NIGP139152, paratype, dorsal external mould (H1) and latex cast (H2); note also ventral interarea. I. NIGP139153, paratype, broken dorsal internal mould. J. Strophomenid gen. et sp. indeterminate; NIGP139154, dorsal internal mould (J1) and latex cast (J2), together with a dorsal internal mould of Leptellina spatiosa sp. nov. (top).

Fig. 2. Middle Ordovician sedimentary facies on the Yangtze Platform, South China palaeoplate (modified from Zhang et al. 2002), with three fossil localities discussed in the text: 1, Shihtzupu Formation at Shizigou Valley near the Weixin County town, Yunnan Province; 2, Type area of Shihtzupu Formation at Donggongsi, Zunyi, Guizhou Province; 3, Dashaba Formation at Shuanghe, Changning, Sichuan Province.

Fig. 1. Location map of the study area. A. Map of China, with the South China palaeoplate outlined by thick line; inset box enlarged as B. B. The border region between Yunnan, Guizhou and Sichuan provinces; inset enlarged as C. C. Detailed map of the Shizigou Valley section near the Weixin County town, with the fossil locality marked by solid triangle.